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Priority of stereoisomers

The method to name and distinguish two stereoisomers was created by three chemists: R.S. Cahn, C. Ingold and V. Prelog. This set of rules, often addressed as Cahn-Ingold-Prelog rules, distinguishes the enantiomers based on their optical ability to deviate light and is often referred to as R/S optical rotation system. By assigning priority to the different groups connected to the central carbon, a stereoisomer can be classified as S if the arrow from the highest priority group to the second-highest moves in a counterclockwise direction, otherwise is classified as R if the arrow moves in a clockwise direction.

Before assigning these configurations, it is necessary to understand that priority is based on the atomic number of the atoms directly connected to the central carbon. A substituent with a higher atomic number takes precedence over a substituent with a lower atomic number and therefore hydrogen has the lowest priority.

The steps to assign R/S configurations are the following:

  1. Assign the priority to the groups based on the atomic numbers of the atoms directly connected to the central carbon.

  2. If two or more of the atoms directly bonded to the carbon are the same, then assign priority based on the next set of atoms adjacent to the directly-bonded atoms, repeating the same procedure until there is a difference.

  3. If two atoms have substituents of the same priority, a higher priority is assigned to the atom with more of these substituents.

  4. Rotate the molecule in space in a way that the group with the lowest priority is directed away from you.

  5. Assign the configuration S if the groups are arranged in a counterclockwise direction, R if they are arranged in clockwise order.

A diagram explaining the three stages of how to determine the R configuration of R-1-bromo-1-fluorethane.The first stage on the far left shows the molecule by itself. The second stage is to assign the priority to the substituents based on their atomic number. Bromine has the highest priority followed by fluorine, then the methyl group and finally the lowest priority hydrogen. Each functional group is highlighted and circled in different colours based on their priority. The third step is to assign the configuration, in this case, the priority order of the groups moves clockwise as bromine is on the right, fluorine on the left, and methyl on top of the molecule and the hydrogen behind the molecule. The clockwise direction is demonstrated with a curved arrow and therefore means that this structure has an R configuration.

Figure 1. Prioritization of stereoisomers.